1
general::
Also, tough nut to crack. A difficult problem; also, an individual who is difficult to deal with. For example, This assignment is a hard nut to crack, or It won't be easy getting her approval; she's a tough nut to crack. This metaphoric expression alludes to hard-shelled nuts like walnuts. [Early 1700s]
American Heritage Idioms
2
general::
Phrase(s): a hard nut to crack [and] a tough nut to crack
Fig. difficult person or problem to deal with. • This problem is getting me down. It’s a hard nut to crack. • Tom sure is a hard nut to crack. I can’t figure him out.
McGrawhill's American Idioms And Phrasal Verbs